- January 30, 2016 at 10:20 pm #22434
While we all usually refer to the bullet in weight truly it is length that matters.
Montana Bullet a cast bullet manufacter has a guide for twist to bullet length that some might find handy at some time
Maximum Length of Bullet Stabilized (inches)
This May or May not be handy to some just thought it might be useful to some as a guide.
For those that already know sorry about posting the obvious
- January 30, 2016 at 10:54 pm #22435GoodsteelKeymaster
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Good post Sarge.
- January 30, 2016 at 11:59 pm #22449DeadWoodDanParticipant
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Very interesting, first I have seen it, but I still have alot to learn. thanks for posting; will be useful as I hope this summer to be filled with data. First chronograph coming, CED millennium!
- January 31, 2016 at 2:27 am #22461
The chart I listed above in the 1st post is utilizing the Greenhill Formula here is some of what was said on that page;
“The Greenhill Formula does not, however, take into account bullet shape or muzzle velocity. Both parameters come into play especially when you approach the limit of what a caliber/twist rate combination should stabilize.
Consider a bullet as a cylinder. A round-nosed bullet is more like a cylinder, has its weight very evenly distributed along its entire length and takes less spin to stabilize. On the other hand, a spitzer-shaped bullet has most of its weight contained within the base half of the bullet. The center of mass is therefore not the middle of the bullet and it will require more spin to keep the nose and base aligned. The moral of the story? When you approach theorized limit of what a caliber/twist rate combination should stabilize, stay away from spitzer-shaped bullets. That is, unless you can keep velocity high. Every shooter knows the term velocity. The reloading manuals are full of ballistic tables showing velocity at the muzzle and 100, 200, 300, etc. yards down range. I’ll bet, however, that most shooters don’t realize that velocity and twist rate determine the speed at which a bullet spins. That spin rate, in revolutions per second or minute, keeps the base and nose of the bullet traveling in the same direction. For example, let’s consider the same 45 caliber rifle with a 1 in 18″ twist rate. At a muzzle velocity of 1,000 feet per second (fps), the bullet is spinning at 666.67 revolutions per second (rps) or 40,000 revolutions per minute (rpm). At a muzzle velocity of 2,000 fps, that same bullet will be spinning at 1333.3 rps or 80,000 rpm. By contrast, take a 22-250 varmint rifle with a 1 in 12″ twist firing a bullet at 3,600 fps. That bullet is spinning at 3,600 rps of 216,000 rpm. The moral of the story? When you approach the theoretical limit of what a caliber/twist rate combination should stabilize and your bullets are keyholing on the target, increase the muzzle velocity (if you can and still stay safe), switch to a more round-nosed bullet shape, or switch to a slightly lighter/shorter bullet.
Use these tables as a general guideline only. I tell all of my long-range shooting customers, shoot the longest, heaviest bullet that your rifle shoots best. Accuracy is more important in the long-range game than muzzle velocity or bullet shape. And yes, Virginia, you must experiment with many different bullets in order to find the one your rifle likes. And that’s why we have the selection we do!”
( http://www.montanabulletworks.com/page11.html )
Another Stability bullet stability formula that is based off Greenhill’s work and modified, is the Miller formula which uses S.G factor. Miller states that a bullet is marginally/barely stable at a factor of 1.4 and fully stable at a factor of 2.0 anything more is overspin. In order to use the use the Miller formula correctly (which I add it is highly effective with Boat tail and is consider the industry standard ) we need to know bullet specifics. Wouldn’t it be great to know all those little factors? Well we are in luck (http://www.shootforum.com/forum/bulletdb.html) my my my a database does exist. The database does contain some common cast bullets such as the Lyman and RCBS. Now that we are armed with that knowledge lets find a calculator many people use Berger’s calculator which is fine but I have noticed that Berger’s is not exactly like Miller’s or every other online source so I generally use JBM’s calculator (http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi) the added plus is that JBM’s site offers quite a few more functions such as long range ballistics. Berger’s online calculator seems to have a tendency to twist a little faster than Miller original formula. Not arguing one against the other just pointing out the subtle difference’s as when I quote JBM always always somebody will come back with what Berger’s site says.
Miller’s formula account’s for more than Greenhill’s formula which was originally done for artillery. Temperature is a factor playing with Miller’s formula we find that velocity has little effect on the stability, but Twist and Temperature / Altitude does. Both are effective but the Miller is better as it accounts for more variables and allows a shooter to calculate for those.
so comparing Greehill’s formula against Miller lets look at what is needed for a 13 twist .308 measuring 1.09″
we will use 2750 as a muzzle velocity because MOST 30 caliber can easily reach this speed.
Stability Input Data Caliber: 0.308 in Bullet Weight: 175.0 gr Bullet Length: 1.090 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2750.0 ft/s Barrel Twist: 13.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 2.095
We see a SG factor of 2.095 what if we go to a slower twist? say a 1-16
Stability Input Data Caliber: 0.308 in Bullet Weight: 175.0 gr Bullet Length: 1.090 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2750.0 ft/s Barrel Twist: 16.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 1.383
Now we see that this is approaching marginally stable at a factor of 1.383 what about a 1-17″
Stability Input Data Caliber: 0.308 in Bullet Weight: 175.0 gr Bullet Length: 1.090 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2750.0 ft/s Barrel Twist: 17.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 1.225
Once you go to the site you will see that the color is yellow…. so the slowest I would twist a rifle for this bullet would be 1-16″.
Another factor that will allow the 1-17″ twist to hit a green color would be temperature noticed I stuck with 59 degrees which is the default. If I drop the Temperature the S.G. goes down.
- February 6, 2016 at 4:10 pm #22969
I purposely wrote this sort of fragmented in talking about stability with cast because it is not exactly easy to just point out the answer to someone whom has not played with it extensively.
Is the factory default in a twist bad? NO they are usually twisted for the longest jacketed projectile (not weight but length) to be a do it all. But, if you are either wringing out the upmost accuracy or the highest accuracy at a way faster velocity than “normal” loads say 2700 fps.
Now our problem lies in the fact that our cast bullets have a shorter length than the Jacketed for the same weight, also the miller formula is the most accurate with boattail which are longer.
But the Miller formula is actually better than the Green hill across the board. so let’s play with some Cast bullet calculations using the Miller formula.
The First thing we must know is our bullet dimensions Now we must either look up that on the mold makers site such as Al Nelson provide or we can use a source such as a online bullet database (http://www.shootforum.com/forum/bulletdb.html ) once we have that up we know that Lyman (LYM) nor RCBS produce jacketed bullets so those are cast bullets even the mold number are Identified. (if your interest is a different caliber than 30 just select the Caliber you desire I choose 30 as a teaching point)
Calibre Manufacturer Bullet Stated Weight True Weight Length SD BC above V1 BC above V2 BC above V3 BC above V4 BC below V4 Velocity 1 Velocity 2 Velocity 3 Velocity 4 0.308 LYM LSp GC 311359 115 118 0.72 0.178 0.181 0.182 0.182 0.182 0.182 2000 1600 0 0 0.308 LYM LFN GC 311441 117 120 0.7 0.181 0.143 0.152 0.163 0.163 0.163 2200 1600 0 0 0.308 LYM LRN GC 311576 120 123 0.72 0.185 0.172 0.196 0.22 0.22 0.22 1800 1400 0 0 0.308 LYM LRN LOV GC 311465 122 125 0.73 0.188 0.163 0.171 0.195 0.195 0.195 2200 1600 0 0 0.308 LYM LRN 311410 130 130 0.86 0.196 0.239 0.25 0.265 0.265 0.265 1600 1200 0 0 0.308 LYM LBN GC 311440 147 150 0.82 0.226 0.134 0.151 0.171 0.171 0.171 2200 1600 0 0 0.308 LYM LRN LOV GC 311466 152 155 0.92 0.233 0.25 0.25 0.26 0.26 0.26 2000 1600 0 0 0.308 LYM LRN GC 311291 170 173 0.95 0.261 0.202 0.231 0.25 0.25 0.25 2200 1600 0 0 0.308 LYM LFN GC 31141 173 176 0.94 0.265 0.22 0.25 0.28 0.28 0.28 2200 1600 0 0 0.308 LYM LSp LOV GC 311467 177 180 1.09 0.271 0.32 0.305 0.3 0.3 0.3 2200 1600 0 0 0.308 LYM LFN GC 311407 180 183 1.02 0.276 0.27 0.3 0.325 0.325 0.325 1800 1400 0 0 0.308 LYM LSp GC 311334 190 193 1.14 0.291 0.34 0.312 0.275 0.275 0.275 1800 1400 0 0 0.308 LYM LRN GC 311299 202 205 1.15 0.309 0.377 0.358 0.39 0.39 0.39 1800 1400 0 0 0.308 LYM LRN GC 311290 210 213 1.19 0.321 0.305 0.3 0.275 0.275 0.275 1800 1400 0 0 0.308 LYM LRN GC 311284 214 217 1.2 0.327 0.332 0.331 0.335 0.335 0.335 1800 1400 0 0 0.308 RCBS SP GC 30-115-SP 115 115 0.72 0.173 0.175 0.175 0.175 0.175 0.175 0 0 0 0 0.308 RCBS SPSpitz GC 7.62-130-SPL 125 125 0.93 0.188 0.3 0.3 0.3 0.3 0.3 0 0 0 0 0.308 RCBS FN GC 30-150-FN 148 148 0.82 0.223 0.22 0.22 0.22 0.22 0.22 0 0 0 0 0.308 RCBS SP GC 308-165-SIL 165 165 0.99 0.248 0.29 0.29 0.29 0.29 0.29 0 0 0 0 0.308 RCBS SP GC 30-180-SP 177 177 1.07 0.267 0.29 0.29 0.29 0.29 0.29 0 0 0 0 0.308 RCBS FN GC 30-180-FN 180 180 1 0.271 0.255 0.255 0.255 0.255 0.255 0 0 0 0
I did not include NOE or other custom mold maker bullets only because they was not in the list but never fear they usually post their drawing. so you can get the length so we are good to go. now look up the bullet and annotate the length and weight. remember that length is more important than weight in the formula.
Goodsteel and some others will always reference Berger stability calculator (http://www.bergerbullets.com/twist-rate-calculator/ ) I dispise it and never recommend it, I prefer to use JBM ballistics stability calculator ( http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi). Why you ask? people are people they usually are lazy and pick the simple actions and then use the Libary on berger site ( remember berger bullets are longer than cast and other jacketed bullet 99% of the time). JBM forces you to plug the number is as no library is available. but I will say IF you do plug the numbers in you will get the same (very very close) results.
So let’s compare some bullets Im pickin the Lyman LRN LOV GC 311466. 155grs weight (caution alloy dependant) at 0.95″ I want to shoot at 2700 fps for example in a ten twist
Stability Input Data Caliber: 0.308 in Bullet Weight: 155.0 gr Bullet Length: 0.950 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2700.0 ft/s Barrel Twist: 10.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 4.600
Ok our stability is a SG factor of 4.6 now to optimize our SG (gyroscopic stability) we need to be 1.4/1.5 on the marginal side to 2.0 but definitely less than 3 regardless of bullet construction we see that we have a whopping 4.6 yeah a bit overspin there lets compare that to say a 210 gr lyman LRN GC 311299. 1.15 length at 210 grs same speed same twist.
Stability Input Data Caliber: 0.308 in Bullet Weight: 210.0 gr Bullet Length: 1.150 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2700.0 ft/s Barrel Twist: 10.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 3.623
Better but still not exactly where we want it lets shoot for 1.8 to 2.3 Caution here as it gets colder we need to spin a little faster so I’m leaving the temperature at 59 degrees if you need to lower it for hunt plug the coldest numbers in and shoot for 1.5ish to 2.0. Twist rate, temperature and altitude will make the biggest difference not velocity ( it take a tail load of speed to make up for the other factors)
Now let’s assume I bought a factory 1-12″ twist rifle and redo do the 210gr Lyman first.
Stability Input Data Caliber: 0.308 in Bullet Weight: 210.0 gr Bullet Length: 1.150 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2700.0 ft/s Barrel Twist: 12.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 2.516
Better much better but still needs refinement.
just for giggles lets just try the 13″ and 14″ for giggles.
Stability Input Data Caliber: 0.308 in Bullet Weight: 210.0 gr Bullet Length: 1.150 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2700.0 ft/s Barrel Twist: 13.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 2.144
hey hey now the 14″ twist
Stability Input Data Caliber: 0.308 in Bullet Weight: 210.0 gr Bullet Length: 1.150 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2700.0 ft/s Barrel Twist: 14.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 1.848
HMMMMM interesting but what about our Lyman LRN LOV GC 311466
Stability Input Data Caliber: 0.308 in Bullet Weight: 155.0 gr Bullet Length: 0.920 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2700.0 ft/s Barrel Twist: 12.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 3.495
Stability Input Data Caliber: 0.308 in Bullet Weight: 155.0 gr Bullet Length: 0.920 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2700.0 ft/s Barrel Twist: 13.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 2.978
Stability Input Data Caliber: 0.308 in Bullet Weight: 155.0 gr Bullet Length: 0.920 in Plastic Tip Length: 0.000 in Muzzle Velocity: 2700.0 ft/s Barrel Twist: 14.0 in Temperature: 59.0 °F Pressure: 29.92 in Hg Output Data Stability: 2.568
as we can see the 1-14″ twist is pretty close. a 1-16″ would probably be the cat’s meow
A high SG factor will induce Spin drift now that is not critical in the jacketed world until long range is involved which 300 is Mid range soo usually anything beyond 6oo is considered long range but that number is also subjective 300 can be long range for a 22 LR. I submit that because we are playing with cast and it is more easily influenced by factors than jacketed that the Spin drift will couple with ***gasp*** can I say it here (RPMTH) I would lower the ranges for long range to beyond 200 yards beyond that the factors will play hard on the cast bullet. Now can that cast bullet travel to 600 to 1000 yard yes sure can, but my jacket 30-06 load can travel 3 miles apples and oranges.
If I was setting up for PURE cast HV shooter I would based on the miller formula and what I have been taught over the years I would probably go to a SG factor of 1.5 to 2.0 that probably net me a 1-16″ or slower twist with a 0.92″ bullet.
But just for giggles look at Greenhill for .920″ length bullet hey it’s the same for atmospheric standard factors. But the Miller will give you the ability to adjust for areas that are not at atmospheric standard such as Dever Colorado the burning sand of Saudi Arabia Afghanistan
- February 6, 2016 at 4:36 pm #22972
As with numerous computer generated programs we will find differences between the various programs. For example I use the Miller Stability Formula. The Sg factors differ from those posted above. The outcome is basically the same we just have to understand the slight differences in the listed Sg factor is all. In the MSF an Sg of 1.4 means the bullet is fully stable. I will quote him chapter and verse from his very fine book; APPLIED BALLISTICS FOR LONG-RANGE SHOOTING, 2nd ed.
So let’s take a look at what Bryan Litz says in that very fine book that relates to bullet stability and precision (accuracy): Chapter 10, Bullet Stability, page 144;
“The relationship between stability and precision [what we generally refer to as “accuracy” or group size] is probably where the bulk of stability misconceptions exist. Gyroscopic stability can affect precision in two ways. First if the bullet is not adequately stabilized, it will emerge from the muzzle and fly with some significant amount of yaw until it stabilizes (goes to sleep).This situation is bad for precision and adds significantly to the bullets drag. This problem can be simply solved by choosing a proper twist rate.
It’s also possible for precision to be compromised if the bullet is spinning faster than it has to be for adequate stability. When a bullet emerges from the muzzle of a rifle, it’s spinning very fast. Any imperfection in the shape, balance or alignment of the bullet will cause it to disperse away from the bore line when it exits the muzzle. The amount of dispersion is related to how severe the imperfections in the bullet are, and also how fast the bullet is spinning. Higher spin rates produce more dispersion. This situation can create the illusion that the bullet’s dispersion is caused by excessive stability, but that’s not the reality. The actual stability level of the bullet is not what causes the dispersion! The imperfections in the bullet cause dispersion, and the dispersion is increased the faster the bullet is spinning. The more balanced the bullet the bullets are, the less dispersion will result from spinning them faster. One of the reasons why short range Benchrest shooters choose to shoot short, blunt, flat based bullets is because they require such a slow twist to stabilize. The slower twist barrels aggravate the imperfections of the bullets much less than faster twist barrels and smaller groups result.
The relationship between spin rate and dispersion is stronger for low quality bullets. This is why British Commonwealth ‘Target Rifle’ shooters long used 1:14″ twist barrels as until recently the discipline’s rules required the use of ‘as issued’ 7.62mm NATO military ammunition which was loaded with relatively poor quality 145 grain FMJBT bullets. As precision bullet manufacturing technology matures and the resulting products move ever closer to having perfect balance, the relationship between spin rate and dispersion is diminishing.
There is much debate about what barrel twist rate or bullet RPM is optimal for precision shooting. The answer isn’t as complicated as it’s made out to be. The right way to choose the proper twist rate for a particular bullet is to select a twist that for a particular bullet is to select a twist that results in a stability factor (Sg) of at least 1.4 [Miller Stability Formula] at your intended velocity. If the stability factor is a little higher, that’s ok. The effects of over spinning a bullet are not nearly as bad as under spinning it. If the Sg is up near 2.0, that’s not necessarily bad. As long as you’re shooting good bullets there will be no detectable difference in precision. If you get a batch of bad bullets, a slower twist barrel may be more forgiving, but the difference would be hard to resolve.”
Understanding how a bullet is stabilized and noting the difference between stability of the bullet and precision (accuracy) of the bullet we get a better understanding why we want to control the RPM of the cast bullet. Of course with a given barrel such as a 10″ twist 30-06 we quickly find that it is easy to maintain the same precision as with a longer 14 or 16″ twist barrel by simply matching the Sg figures. What we find in doing so is the precision can be the same and the Sg factors can be the same but the velocity difference will be remarkably different. The velocity level will be much lower with the 10″ twist barrel as we see with the chart Sgt. Mike has posted.
- February 6, 2016 at 4:54 pm #22976
OOOOOO and just for the record the 16″ battle ship guns are twisted 1-25 CALIBERS not in inches so 16″ shell time 25 = 400″ divide by 12 that is 1turn in 33 feet the gun tube is 66 feet long.
I hate lairs.
- February 6, 2016 at 4:55 pm #22977
OOOOOO and just for the record the 16 inch battle ship guns are twisted 1-25 CALIBERS not in inches so 16 inches shell diameter times 25 = 400 inches divide by 12 that is 1turn in 33 feet the gun tube is 66 feet long.
I hate lairs.
- February 6, 2016 at 5:00 pm #22979
Now that we have revisted Larry’s and my conversation of over what Larry a year or more ago?
Hopefully this helps explain to some why some are going the slow twist route.
- February 6, 2016 at 5:03 pm #22981
Seems it was well over a year ago to me but then ……..
- February 6, 2016 at 5:15 pm #22982
yes yes it was Larry……. LOL I believe it was over the exact same topic, Yet I also recall we got into casting consistency and way more ( here is where a plug should be inserted on Goodsteel consistency applied thread on CBF)
- February 6, 2016 at 5:28 pm #22985
Here’s some examples fo what can be done with a 10″ twist .308W. Now this test rifle is an Argentine made M1909 action with a Midway $89 Adams & Bennett barrel on sale. I installed the barrel and finished reamed with a Milspec M118 match finish reamer. It is a 24″ sporter with 10″ twist. It is bedded in an old Churchill Arms (England) sporter wood stock. It does shoot MK 168 and 175s into right at moa with 10 shot groups. Scope is an old original Weaver K-10. Nothing really special about the rifle other than to note it is just a good “sporter”.
When you push the RPM Threshold upwards you can do pretty much as these different loads did. The RPM Threshold is still there though. I just pushed it higher. I did not cross it with these loads but if I push up to 2450+ fps the RPM threshold with this rifle using this bullet with these powders is then crossed. The accuracy goes south rapidly giving 2.5 – 3.5″ groups. However by casting a properly designed bullet of the correct alloy correctly, then carefully sorting the bullets and loading them correctly we can push up the RPM Threshold as shown here.
The MSF Sg for the 46 gr RL19 load is 4.25 which is really spinning those suckers fast!
Obviously I’ve not got how to properly insert attachments figured out yet…..:(
- February 6, 2016 at 6:59 pm #22993
However by casting a properly designed bullet of the correct alloy correctly, then carefully sorting the bullets and loading them correctly we can push up the RPM Threshold as shown here
Huh kinda sounds like a handloader versus a reloader…
well I think I’ll go and cast some bullets see yall later gents
- February 6, 2016 at 7:47 pm #23003shdwlkrParticipant
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Good thread, I tend to select the bullet and twist and then adapt it to the cartridge, case in point I am having a .225 winchester also have a .256 win mag being built with a 1 in12 twist I like the twist and am willing to play around and find what works best in each, if it is jacketed so be it, if it is lead so be it if it is both yea. I think to often we want to get the most speed we can from a given cartridge and that just might not be the most accurate. I will take an accurate load to one that approaches the the speed of light and you need a big barn to hit anything any day.
- February 6, 2016 at 9:34 pm #23021GoodsteelKeymaster
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For the record, I posted “Consistency applied” in the Articles section of Goodsteelforum.
- February 7, 2016 at 12:48 am #23045Rossi.45Participant
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thank you for the very interesting information . . . very pleased to find this forum
- February 9, 2016 at 6:11 am #23387AnonymousInactive
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Very informative thread. Thanks for all the posts gentlemen.
- February 9, 2016 at 7:51 pm #23418HarterParticipant
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This has been interesting. It explains why I have gotten results that are in some cases impossibly different from the results of others . I would sight a 308 that shoots 145 gr fmj bt and 150 gr BTSP but won’t shoot a 180 gr RN of identical length .. The chart references temperature and pressure. Very valid points in my difference in results . Every day my uncorrected barometric pressure for a standard day is 27.4 vs 29.92 ,I suspect that makes a substantial difference in the computation s.
- May 15, 2016 at 4:31 am #27827
From Sgt Mike’s initial post:
“While we all usually refer to the bullet in weight truly it is length that matters.
Montana Bullet a cast bullet manufacter has a guide for twist to bullet length that some might find handy at some time”.
I got a question on weight vs length in estimating twist or stability.
I’ve got a flat point .410 diameter bullet for which I’d like to know more about stability in a 20:1 twist out to 800yds. If a fellah has a money bullet vs a flat point the length would be considerably different. The Montana chart shows a length of 1.25, but would that change if a bullet was a flat point vs a money bullet profile? And if so, do we start to scrutinize weight more? At 1.18 in length I’m within the chart recommendations with my length for a 20:1 twist but if I understand the center of gravity correctly the flat nose would be in even safer territory than a money bullet of equal length. My bullet weight is 360 and my velocity is just under 1400fps to round out the information. I wouldn’t be concerned if it was not for the 800 yd distance.
- May 15, 2016 at 5:38 am #27829
What are the lengths on the two different bullets with the checks applied (if provided for in the mold design).
As long as the bullet stays under the 1.25″ listed it will maintain stability, I think that your mentioning range of 800yards you are thinking that the rate of rotation would be greatly reduced.
Which one will shoot more accurately I dunno go shoot the two against each other at your desired range and distance as according to the length you mentioned both will be stable.
Well some folks (US Military) did do a study of rotational decay which was noted to be extremely minimal (at 800 yard- 89% appox. of muzzle RPM was retained , that is only 11% loss).
I think you would see more of a issue of linear dispersion which Larry Gibson gets far better into that explanation than I do.
But stability and accuracy are two different things……… stability means my bullet is not tumbling sideways, or travelling with a substantial amount of precession, or yaw as to tip or act erratic when striking a object.
Now that I made that statement if I have instability my cone of fire is double or even triple of a stable bullet path which will assure inaccuracy. in order to have a degree of accuracy ( that degree can be 1 MOA, 2MOA, 3MOA, 4MOA) I have to have stability so yes they are joined at the hip so to speak
- May 15, 2016 at 4:43 pm #27839
Thanks Sgt Mike… Especially for the 89% RPM retention figure. Got a field located and shooting hopefully this week. Just exploring particulars in the mean time and getting familiar with some of the formulas. Jotted these down last night:
Using sqrt of velocity for constant
130x.41Ox.410/1.18 =18.5 twist.
Using 150 for the constant
150x.410x.410/1.18 =21.3686 twist.
Using Shaver’s 135 for a constant
135x.410x.410/1.18 = 19.23 twist
It’s a Ballard Pacific in 40-63 and I’m taking it to Quigley.
I’ve got two molds for it. One drop a 360gr at 1.18 length. Another drops at 317gr but is only .98 long and originally for a vintage Marlin 1895 in 40-65. Shoots very well in the Ballard though.
Saw this quote from Goodsteel on Chris’ mold question thread: “If a projectile is made of aluminum, and an identical projectile is made of tungsten carbide, they will be vastly different in weight, but they will both require the same RPM to stabilize them in flight”, so that helps too.
- May 15, 2016 at 11:40 pm #27852
I took a stab at the BC (.15) and with the other info the Sg came out at 1.96. Obviously the bullet is stable. If you provide some other info I can give you more complete ballistic data?
Sight height above center of bore;
- May 18, 2016 at 8:08 pm #27887
Thanks Larry, Here is what I know:
BC; 3166 – that according to Tom Myers application. I designed the bullet with that software although not for this kind of shooting. It’s what I got though, unless I default to a little 315gr bullet I have with a similar profile and a BC of 3079. So far that bullet does shoot a tighter group for me. I was sticking with the heavier of the two thinking heavier would be best for wind since it also had a better BC. Got to make up my mind soon since I’ll start loading tomorrow.
Max range; 800 for this event
Temp; probably be warm in Forsyth come June
Humidity; unknown. I’d guess fairly dry by mid-June.
Wind speed; unpredictable – hopefully not bad but frequently now good. I’d guess I’d be safe counting on 20mph. The Quigley is shot on wide open Montana prairie.
Sight height above center of bore; Crosshairs in the globe are .774 above center of bore.
Zero range; 50 yds
Note about my choice of gun and bullet. A lot of this was driven by timing. I intended to use a relined Ballard in 40-65 with a 15 twist and a 400 gr money bullet. But my reamer order from Manson seems to have got filed in the nether-world and John Taylor can’t very well get started without the reamer. Woulda taken another path had I paid more attention to the calendar and settled on an off-the-shelf reamer.
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